A silicon dioxide layer which is commonly used as an isolating or passivating layer, can be generated on a silicon surface for example by thermal oxidation. Thermal oxides are grown when a silicon layer is held at a temperature between 700° C. and 1200° C. in an oxidizing atmosphere. In particular, thick field oxide layers which are used for integrated MOS circuits can be produced by the LOCOS (Local Oxidation of Silicon) process wherein the portions on which the silicon dioxide layer is not to be grown are covered by a masking layer such as made of silicon nitride.
On the one hand, the growth rate of thermal oxides usually depends on one side on the crystal orientation of the underlying silicon layer. On the other hand, if thermal oxides are to be grown on curved surfaces, the growth rate also depends on the direction and the amount of the curvature. Since during thermal oxidation the volume of the former silicon layer increases due to the interstitially embedded oxygen atoms, stress which is dependent on the specific curvature is induced, which in turn, will result in an inhomogenous growth of the silicon dioxide layer thickness.
As a consequence, only inhomogenous layer thicknesses are achieved on curved silicon surfaces.
In the past, this problem has been dealt with by thermally oxidizing only non-curved or equally curved silicon surfaces. Alternatively, the thickness of the grown silicon dioxide layer has been chosen so that even the thinnest area has the desired thickness. In addition, in the manufacture of devices demanding for homogenous oxide layer thicknesses, the silicon dioxide layer has been produced on curved silicon surfaces by a deposition method in which the growth rate does not depend on the curvature.
One example wherein it would be highly desirable to thermally grow a homogenous silicon dioxide layer on a curved silicon surface is the formation of the collar region in the upper part of the trench of a DRAM (Dynamic Random Acess Memory) cell.
Such a memory cell comprises an access transistor as well as a storage capacitor for storing a charge representing data in the form of either a logical “1” or a logical “0”. When the data is read from the memory cell, sense amplifiers detect the level of the charge stored on a particular capacitor so as to produce a logical “1” or a logical “0” output based on the stored charge.
The capacitor can, for example, be formed in a trench in the semiconductor substrate. The trench is filled with a dielectric and has polysilicon acting as the top electrode of the capacitor. In order to suppress the formation of a parasitic transistor in the upper part of the capacitor trench, it is necessary to provide an isolation collar, for example, of silicon dioxide in the upper trench portion. This isolation collar has a length of approximately 1 μm and a thickness sufficient, i.e., approximately 25 nm, to suppress the transistor action. Since the trench usually assumes an oval cross section, the silicon surface is curved, which makes a uniform thermal oxide growth according to conventional methods difficult.